In fabrication of semiconductor elements, there is a demand for the formation of a finer pattern by lithography in order to increase the degree of integration. As a method of the miniaturization, a source of radiation (light) having a short wavelength (hereinafter, light is also regarded as a form of radiation) is indispensable, and in addition to conventional g-line and i-line, far-ultraviolet rays such as krypton fluoride (KrF) excimer laser (wavelength of 248 nm) and argon fluoride (ArF) excimer laser (wavelength of 193 nm) have been introduced in mass-production. Further, there also have been studies on lithography techniques in which fluorine dimer (F2) excimer laser (157 nm), extreme ultraviolet rays (EUV), electron beam (EB) or the like is used as the radiation source.
A composition for lithography is used in these lithography techniques to form a resist film for transferring a pattern onto a substrate. In addition, a composition for lithography may be used as required to form various thin films above or below a resist film. These compositions are solutions in which polymers and additives having respective target functions are dissolved in an organic solvent. The composition solution is dispensed onto the substrate by a method such as spin coating and the resultant is then heated to remove the solvent, thereby forming a thin film for lithography.
The compositions for lithography used in the formation of a resist film (hereinafter, may be referred to as “resist film forming agent”) are classified into positive type whose part exposed to radiation dissolves in a developer and negative type whose part not exposed to radiation dissolves in a developer. A type which comprises a compound whose solubility in a developer changes by the action of radiation and polymers soluble in an alkaline developer, and a type which comprises a compound that generates an acid by the action of radiation and copolymers whose solubility in an alkaline developer changes by the action of an acid are known. The latter type is specifically called chemically amplified resist and among such resists, chemically amplified positive resists are especially preferably used in microfabrication.
As a composition for lithography to form a thin film above or below a resist film, for example, an anti-reflection film forming agent which is applied onto high-reflection substrate surface (below a resist film) and/or the surface of a resist film (above a resist film) to reduce the reflection in the resist film interface and to suppress a standing wave, thereby precisely forming a fine resist pattern; a gap-fill film forming agent which is applied onto the substrate surface (below the resist film and/or anti-reflection film) when a resist pattern is further formed on the substrate onto which a pattern has already been formed, thereby filling the gaps on the substrate surface for planarization; and topcoat film forming agent which is applied onto a resist film to suppress the penetration of the immersion solution into the resist film and the elution of the radiation-sensitive acid generator and the like in liquid immersion photolithography are known.
In the above-described composition solutions for the formation of a thin film, copolymers for lithography are important constituents in which optical properties, chemical properties and physical properties are demanded for the expression of respective functions of thin films, and they are now widely studied. For example, for chemically amplified positive resist film forming agent which utilizes KrF excimer laser, copolymers having a repeating unit originated from hydroxy styrene and a repeating unit in which phenolic hydroxyl group originated from hydroxy styrene is protected by a group such as an acetal structure or tertiary hydrocarbon group, which group inhibits dissolution in an alkaline developer and also dissociates by the action of an acid (hereinafter, may be referred to as “acid-dissociable dissolution-inhibiting group”), or a repeating unit in which carboxyl group originated from (α-alkyl)acrylic acid is protected by an acid-dissociable dissolution-inhibiting group such as an acetal structure or tertiary hydrocarbon group, are known (see, for example, Patent Literatures 1-4). Further, copolymers having a repeating unit in which alicyclic hydrocarbon group was made an acid-dissociable dissolution-inhibiting group in order to improve the resistance to dry etching and the difference in the dissolution rates of copolymers in an alkaline developer before and after the exposure are known (see, for example, Patent Literatures 5-6).
As for chemically amplified positive resist film forming agent which utilizes ArF excimer laser, copolymers not having a repeating unit originated from hydroxy styrene, which has a high extinction coefficient for the wavelength of 193 nm, have been examined. Copolymers having a lactone structure in the repeating unit (see, for example, Patent Literatures 7-10) and those having a hydroxyl group in the repeating unit (see, for example, Patent Literatures 11-14), which lactone structure and hydroxyl group are contained as a polar group to improve the adhesiveness to a semiconductor substrate or the like and to control the solubility in a lithography solvent or alkaline developer, are known.
For anti-reflection film forming agent, the copolymers comprising a repeating unit having an aromatic ring such as benzene ring, naphthalene ring or anthracene ring as a functional group which increases the extinction coefficient and refractive index to the wavelength of 248 nm and 193 nm, and a repeating unit having a reactive functional group as required, which reactive functional group such as amino group, amide group, hydroxyl group or epoxy group can be cured upon reacting with a curing agent or the like to avoid mixing with resist film (see, for example, Patent Literatures 15-18), are known.
For gap-fill film forming agent, copolymers comprising a repeating unit having an appropriate viscosity to flow into a narrow gap and a reactive functional group which can be cured upon reacting with a curing agent or the like to avoid mixing with resist film and anti-reflection film are known. Specifically, the copolymer comprising a repeating unit which is originated from hydroxy styrene and as required, a repeating unit which is originated from a polymerizable monomer such as styrene, alkyl(meth)acrylate or hydroxyalkyl(meth)acrylate is known (see, for example, Patent Literature 19).
For topcoat film forming agent in immersion lithography, the copolymer comprising a repeating unit having a carboxyl group (see, for example, Patent Literature 20), the copolymer comprising a repeating unit having a fluorine-containing group which is substituted by a hydroxyl group (see, for example, Patent Literature 21) and the like are known.
It is not preferred to make these copolymers dry solid during the production of the copolymer since there are effects such as partial elimination of an acid-dissociable dissolution-inhibiting group due to undue heat load on the copolymers. Therefore, a method in which copolymers are re-dissolved in a good solvent having a boiling point lower than that of the solvent for coating film formation without drying the copolymers from the state of wet cake or the like, and the resulting solution is heated under reduced pressure while adding thereto a solvent for coating film formation to distill off a part of the solvent for coating film formation and components having a low boiling point, so that a copolymer solution in which components having a low boiling point are reduced is obtained (see Patent Literature 22); and a method in which the copolymers are re-dissolved in the solvent for coating film formation from the state of wet cake and the resulting solution is heated under reduced pressure to be concentrated, so that a copolymer solution in which components having a low boiling point are reduced is obtained (see Patent Literatures 23-25), are known.
However, it was not known that, in a copolymer solution produced by such methods, the copolymer concentration varies among a plurality of product containers even if the containers are filled with copolymer solution from the same production lot. Therefore, the film thickness and lithography properties of lithography thin film vary when such a copolymer solution is used as a lithography composition, which has been a major obstacle to the demand for further miniaturization of lithography patterns.    Patent Literature 1: JP S59-045439 A    Patent Literature 2: JP H05-113667 A    Patent Literature 3: JP H10-026828 A    Patent Literature 4: JP S62-115440 A    Patent Literature 5: JP H09-073173 A    Patent Literature 6: JP H10-161313 A    Patent Literature 7: JP H09-090637 A    Patent Literature 8: JP H10-207069 A    Patent Literature 9: JP 2000-026446 A    Patent Literature 10: JP 2001-242627 A    Patent Literature 11: JP H07-252324 A    Patent Literature 12: JP 2000-137327 A    Patent Literature 13: JP 2000-330287 A    Patent Literature 14: JP 2001-109154 A    Patent Literature 15: JP 2000-313779 A    Patent Literature 16: JP 2001-27810 A    Patent Literature 17: JP 2001-192411 A    Patent Literature 18: JP 2001-226324 A    Patent Literature 19: JP 2003-57828 A    Patent Literature 20: JP 2006-193687 A    Patent Literature 21: JP 2006-243308 A    Patent Literature 22: JP 2005-173252 A    Patent Literature 23: JP 2006-161052 A    Patent Literature 24: JP 2007-148328 A    Patent Literature 25: JP 2007-146020 A